There are definitely a few bugs in the DA tech trees.  One well known one is the MCC.  Two others I have found are that one of the "Barriers" techs is worse than the one before it and one of the higher weapons (can't remember which one) is completely screwed up (I rarely use that weapon tree, beam, I think, but IIRC you have to research it to get to the next one but you can't put it on a ship).

In DA Suicidal, the old hands had it down.  There are some odd wrinkles possible in naming things like your home system or homeworld that give you an unexpected edge, for example.  I discovered one such independently, but rarely use it.  I win at Suicidal by rejecting bad setups (homeworlds with no useful bonus tiles, for example) and by making sure to bribe the AIs into multiple wars before they declare war on me.  Meanwhile, I rush to get Trade and trade it to everyone I meet.  Once the AIs get into wars, suddenly I am the one of only ones they are trading with so they delay declaring war on me a bit extra.  That and being careful to do a good colony rush yet not over-extend are the basic elements.

I tried going down a notch in difficulty, but the games seemed boringly easy by comparison, so I went back to Suicidal.

In fact I highly recommend it.  

I agree with LTjim, it's not worth the £14.99 just to get the last expansion.  It's only worth it for new players because at least they can play DA if they find TA to be too much of a pushover.

If Stardock were still updating GC2 then it might be worth it, but that's not going to happen any time soon with all the development issues that have been cited.  Anyone who buys the Ultimate Edition is getting an unfinished product.

Let me put it another way.  When the early adopters paid full price for each expansion they were getting a better deal than the folks who pick up the UE now, because if they found a bug that required a hardcode fix there was a chance of it being squished, as opposed to now.

There's only a couple of AI personalities which work properly in TA with regard to expansion.

The Arceans are crippled by their speed penalty.  Super Warrior is a trait which requires fast ships - so that they can get into range and be the aggressor.

In the early part of the game any AI will attempt to overcome its problems by spamming improvements which are supposed to improve the deficient area - mainly production and research.  It also builds ships at every single colony.  Unfortunately by doing this it crashes its economy very early on, halting expansion prematurely.  Even worse than that, it will attempt to make up the deficit by raising taxes, but that actually slows their early population growth down to a crawl.

The early improvement spam is the reason why I cut many of the basic improvements in my mod, created some early Super Projects (can't spam those) and have the improvements unlock in an order which allows for a better economy to be established before all the maintenance and funding-hungry improvements get into play.  The AI still spams ships at any colony it has a starport on but because it has lower maintenance, it can get away with it.  In fact in most games they all do some serious colonising.

Because of some of the fixes to the tech trees, when I take a look to see what the AI is working on I'm hardly ever thinking 'Why the frell do you need to research that?' and I'm more like, 'Okay, that's going to be a problem for me.'

That's a diplocheese tactic though.  The AI can't upgrade those buildings without the necessary technology so it clogs up tiles with those low-yield factories and slave pits.